1. Field of the Invention
This invention relates to increasing the oxidation resistance of molybdenum, its preparation and its use in electric lamps. More particularly, this invention relates to molybdenum coated with silicon nitride and having increased oxidation resistance at temperatures above 350.degree. C. and its use in electric lamps for increasing the life of molybdenum leads and also of hermetic seals between molybdenum and glass in lamps employing such seals.
2. Background of the Disclosure
The use of molybdenum for wire leads and other parts for electric lamps is old and well known to those skilled in the art. Molybdenum is used for such applications because of its ductility, conductivity, refractory properties and its thermal expansion properties which enable it to form hermetic seals with vitreous materials, such as glass and quartz. However, molybdenum is an oxidation sensitive material and rapidly oxidizes in an oxidizing environment (such as air) at temperatures of about 350.degree. C. and higher. In the case of molybdenum used for outer lead wires and for foils for forming a hermetic seal with vitreous materials, such as a glass lamp envelope, this oxidation eventually results in an open electric circuit and lamp failure. In the case of molybdenum foil seals, the passageways or cracks formed during the sealing process permit oxygen to enter the foil area of the lamp seal as is disclosed in U.S. Pat. No. 4,918,353. Past efforts made to prevent the oxidation of molybdenum foils have included coating the outer half of the molybdenum foil with chromium (U.S. Pat. No. 3,420,944) and having the chromium coating wedge shaped (U.S. Pat. No. 3,793,615). Other proposed solutions to preventing oxidation of molybdenum outer leads consist of (i) covering the molybdenum with a coating or sleeve of nickel-plated brass (U.S. Pat. No. 4,015,165) and (ii) applying a sealing glass composition to the small space or passage between the outer leads and the lamp envelope (U.S. Pat. No. 4,539,509).
Notwithstanding the above, a problem still exists with respect to preventing the oxidation of molybdenum wire leads, lamp parts and foils exposed to an oxidizing environment at temperatures above about 350.degree. C.